1. Field of the Invention
The present invention relates to a steel plate for enameling which can be produced at low cost and possess improved (excellent) enameling properties, formability, and anti-aging property, and a process for producing the same.
2. Background Art
Steel plates for enameling have hitherto been produced by decarbonization and denitrification annealing to reduce carbon content and nitrogen content to not more than several tens of ppm. The decarbonization and denitrification annealing, however, has drawbacks of low productivity and high production cost.
In order to avoid decarbonization and denitrification annealing, for example, Japanese Patent Laid-Open No. 122938/1994 and Japanese Patent No. 2951241 disclose steel plates for enameling, using ultra low carbon steels, wherein the carbon content has been reduced to several tens of ppm by degassing at the point of steelmaking. In these techniques, in order to eliminate the adverse effect of carbon in solid solution or nitrogen in solid solution left in very small amounts in the steel, titanium, niobium and the like are added to improve deep-drawability and anti-aging property.
In this method, however, seeds and black speck defects attributable to carbides and nitrides are likely to occur. In addition, the production cost is disadvantageously increased due to alloying cost of titanium, niobium and the like.
Steel plates for enameling with the amount of titanium, niobium and the like added being reduced and a process for producing the same are disclosed, as steel plates and the production process thereof which can solve these problems, in Japanese Patent Laid-Open Nos. 27522/1996, 137250/1997, and 212546/1998, although these plates have somewhat inferior drawability. In these methods, boron is mainly used in the fixation of nitrogen.
In the methods disclosed in the above publications, however, a reduction in carbon in solid solution is not satisfactory under some production conditions. Further, redissolution of nitrides during annealing leads to increased nitrogen which causes age deterioration and thus disadvantageously deteriorates press formability. In addition, disadvantageously, the evolution of gas, for example, due to the decomposition of nitrides during baking of porcelain enamel, is likely to cause seeds and black speck defects.
Accordingly, it is an object of the present invention to solve the above problems of the conventional steel plates for enameling and to provide non-aging steel plates for enameling, which have excellent anti-seed and anti-black-speck properties, can be produced at low cost, and have good formability, and a process for producing the same.
The present inventors have repeatedly made various studies with a view to overcoming the drawbacks of the conventional steel plates and the conventional production process of steel plates. More specifically, the present inventors have made studies on the influence of chemical composition and production conditions on the aging property and enameling properties of steel plates for enameling. As a result, the present invention has been made based on the following findings (1) to (5).
(1) Mere addition of carbide formers is unsatisfactory for the suppression of aging and seeds and black specks, and the absolute value of the carbon content should be reduced to not more than a certain value.
(2) The aging property and the occurrence of seeds and black specks are influenced by the type of nitrides, and the anti-aging property and the anti-seed and anti-black-speck properties are improved by the formation of boron nitride rather than aluminum nitride.
(3) The aging property and the occurrence of seeds and black specks are influenced by the form of boron nitride, and the anti-aging property and the anti-seed and anti-black-speck properties are improved by regulating the content and size of boron nitride so as to fall within respective specific ranges.
(4) Not only the regulation of nitrogen and boron contents but also the regulation of particularly oxygen content and hot rolling conditions is useful for the regulation of the state of the nitride in the above manner.
(5) In steels wherein carbon, phosphorus, nitrogen, and boron contents and the state of nitride have been properly regulated, the optimal skin pass reduction range for maintaining good anti-aging property and formability can be broadened.
The present invention is based on the above facts, and the subject matters of the present invention are as follows.
(1) A steel plate for enameling, having improved formability, anti-aging property, and enameling properties, comprising by mass
carbon: not more than 0.0018%,
silicon: not more than 0.020%,
manganese: 0.10 to 0.30%,
phosphorus: 0.010 to 0.035%,
sulfur: not more than 0.035%,
aluminum: not more than 0.010%,
nitrogen: 0.0008 to 0.0050%,
boron: not more than 0.0050% and not less than 0.6 time the nitrogen content, and
oxygen: 0.005 to 0.050%,
(nitrogen present as BN)/(nitrogen present as AlN) being not less than 10.0,
with the balance consisting of iron and unavoidable impurities.
(2) A steel plate for enameling, having improved formability, anti-aging property, and enameling properties, comprising by mass
carbon: not more than 0.0018%,
silicon: not more than 0.020%,
manganese: 0.10 to 0.30%,
phosphorus: 0.010 to 0.035%,
sulfur: not more than 0.035%,
aluminum: not more than 0.010%,
nitrogen: 0.0008 to 0.0050%,
boron: not more than 0.0050% and not less than 0.6 time the nitrogen content, and
oxygen: 0.005 to 0.050%,
(nitrogen present as BN)/(nitrogen content) being not less than 0.80,
with the balance consisting of iron and unavoidable impurities.
(3) A steel plate for enameling, having improved formability, anti-aging property, and enameling properties, comprising by mass
carbon: not more than 0.0018%,
silicon: not more than 0.020%,
manganese: 0.10 to 0.30%,
phosphorus: 0.010 to 0.035%,
sulfur: not more than 0.035%,
aluminum: not more than 0.010%,
nitrogen: 0.0008 to 0.0050%,
boron: not more than 0.0050% and not less than 0.6 time the nitrogen content, and
oxygen: 0.005 to 0.050%,
the average diameter of precipitates of BN alone or BN-containing composite precipitates having a diameter of not less than 0.005 xcexcm and not more than 0.50 xcexcm being not less than 0.010 xcexcm, not more than 10% of the number of precipitates of BN alone or BN-containing composite precipitates having a diameter of not less than 0.005 xcexcm and not more than 0.50 xcexcm being accounted for by precipitates having a diameter of not more than 0.010 xcexcm,
with the balance consisting of iron and unavoidable impurities.
(4) A process for producing a hot rolled steel plate for enameling, having improved formability, anti-aging property, and enameling properties, comprising the steps of:
hot rolling a cast slab comprising by mass
carbon: not more than 0.0018%,
silicon: not more than 0.020%,
manganese: 0.10 to 0.30%,
phosphorus: 0.010 to 0.035%,
sulfur: not more than 0.035%,
aluminum: not more than 0.010%,
nitrogen: 0.0008 to 0.0050%,
boron: not more than 0.0050% and not less than 0.6 time the nitrogen content, and
oxygen: 0.005 to 0.050%; and
then subjecting the hot strip to skin pass rolling with a reduction of not more than 5%.
(5) A process for producing a cold rolled steel plate for enameling, having improved formability, anti-aging property, and enameling properties, comprising the steps of:
hot rolling a cast slab comprising by mass
carbon: not more than 0.0018%,
silicon: not more than 0.020%,
manganese: 0.10 to 0.30%,
phosphorus: 0.010 to 0.035%,
sulfur: not more than 0.035%,
aluminum: not more than 0.010%,
nitrogen: 0.0008 to 0.0050%,
boron: not more than 0.0050% and not less than 0.6 time the nitrogen content, and
oxygen: 0.005 to 0.050%;
cold rolling the hot strip with a cold rolling reduction of not less than 60%;
after the cold rolling, annealing the cold strip at or above the recrystallization temperature; and
subjecting the annealed strip to skin pass rolling with a reduction of not more than 5%.
(6) The process for producing a steel plate for enameling, having improved formability, anti-aging property, and enameling properties according to the above item (4) or (5), wherein the cast slab is hot rolled at a slab heating temperature of 1000 to 1150xc2x0 C.
(7) The process for producing a steel plate for enameling, having improved formability, anti-aging property, and enameling properties according to any one of the above items (4) to (6), wherein the cast slab is hot rolled, and is coiled at 650 to 750xc2x0 C.